Air laid pads of comminuted wood pulp, or other short length cellulosic fibers such as cotton linters, are often used as constituents of absorbent structures, such as diapers and sanitary napkins. Such pads can also serve in structures where cushioning, alone, is the objective and water absorbency may be neither needed nor desired.
The air laying of dry fibers has a substantial advantage over wet laying for the foregoing purposes because dry fibers are relatively stiff and do not sag against each other as wet fibers do; and the dry fibers therefore produce a bulkier, or higher loft structure. A bulkier structure has a greater potential absorbent capacity for liquids and also a greater cushioning ability.
On the other hand, an air laid mass of fibers is essentially composed of a number of completely loose fibers, free except for friction, to move past and away from each other. There is little, if any, of the "natural" or hydrogen bonding among the fibers which develops when wet cellulose fibers are dried in contact with each other, the bonding which is the basis of papermaking.
An air laid pad is therefore easily broken up when subjected to handling such as would occur if the pad were utilized as an element in a diaper. The pad breaks in some places and bunches up in others. Where it breaks, it forms a gap which impedes the desired spreading of absorbed liquid from one area to another. Where is bunches up, it can be uncomfortable to the wearer.
To minimize the tendency of air laid pads to break up and for portions thereof to shift relative to each other, it is common to enclose an air laid pad in creped paper when the air laid pad is used as the absorbent element in a diaper. This is costly because creped paper is a comparatively expensive material, a separately made finished product in itself, because the creped paper, being a wet laid product, is relatively dense and therefore has little liquid holding capacity per unit weight of fiber, and because of the process complexity involved in the infeed of the creped paper and the wrapping of the air laid pads therewith.
An improved reinforcement system for air laid pads, disclosed in Burgeni U.S. Pat. No. 3,017,304, avoids the use of crepe paper wrapping by providing a densified skin on the pad which is integral therewith. The densified skin is prepared by the application of a water spray to one surface of the pad, compressing the pad, and then permitting the pad to dry. Under the force of compression, the entire pad becomes somewhat more dense, but the densification is substantially more pronounced where the fibers have been dampened and are therefore more limp. Upon drying, the dampened fibers develop hydrogen bonding and become a paper-like layer which blends gradually into the main mass of the pad with no sharp line of demarcation. There are many fibers in the structure which are immobilized at one end in the paper-like layer through hydrogen bonding and which at the other end, though relatively free are still restrained to a degree and capable of restraining other fibers which happen to be entangled among them.
Experience has shown that such densified, paper-like skins effectively prevent breaking and bunching in an air laid pad, particularly when the skin is protected from surface abrasion by a cover of some kind, such as a diaper facing. In addition, the densified skin can be anchored, as in a diaper, to a plastic skin or a fabric, which serves as further reinforcement.
The strength of the paper-like skin is a function of its average density and thickness. In other words, it is dependent on how much of the pad volume has been sacrificed for production of the skin. Furthermore, compression of the pad in the formation of the skin reduces the volume of the pad beyond the limit of the spray penetration and thus reduces its liquid holding capacity.
In an entirely different system used for manufacture of print bonded non-wovens, Drelich and Drelich et al. U.S. Pat. Nos. 3,706,595; 3,720,562; 3,769,067; 3,821,146; 3,849,173; 3,857,728; 3,873,486; and 3,889,024 teach the reinforcement of fibrous structures with resinous systems whose migration in the fibrous structure is limited by the conversion of a relatively free-flowing resinous dispersion to a nonflowing coagulated layer. Among the resinous systems disclosed in the Drelich and Drelich et al. patents are systems which are stable dispersions at certain concentrations but which become unstable and coagulate when diluted. To achieve dilution rapidly, the fibrous structures are pretreated with large amounts of water and this water immediately dilutes the stable dispersion when the latter is applied to a surface of the fibrous structure. Such prewetting makes the fibers limp throughout the pad and tends to compress the pad and reduce its loft.